Post-Soviet countries have experienced a very recent and very rapid rise in HIV infection rates, and parts of the region now have disease incidence among the highest in the world. HIV prevention efforts undertaken now can avert the unfolding of an even greater epidemic. Social network intervention approaches are culturally well-suited to Eastern Europe, a region characterized by dramatic and often difficult socioeconomic and cultural transformations, and where individuals have long relied on their social networks for social support and trusted advice. Over the past 4 years, our international collaborative team has successfully carried out studies evaluating the impact of social network HIV prevention interventions with young men who have sex with men (YMSM), ethnic minority Roma (Gypsies), and young high-risk heterosexual adult (YHA) women and men in Russia, Bulgaria, and Hungary. This competitive renewal application requests support to undertake a new generation of social network intervention research trials with these populations. The study will use a 3-wave snowball enrollment method to recruit 24 social networks of YMSM, Roma, and YHAs, each expected to include approximately 60 persons (n=1,440). All participants will complete baseline network structure and sociometric measures and risk assessment interviews; be offered testing and treatment for STDs and HIV; and receive risk reduction counseling. 20% of participants in each network will be identified as leaders based on their high sociometric standing and high connectedness with others in the network. Networks will then be randomized in equal numbers to experimental and control conditions (n=12 networks, 720 participants per arm). Leaders will attend a 9-session program that trains and guides them in delivering theory-based risk reduction recommendations to their network members. The messages will target network members' risk reduction knowledge, attitudes, intentions, perceived norms, and self-efficacy. Leaders of control networks will attend an attention-matched intervention unrelated to AIDS. At 3- and 15- month follow up, the cohort will be assessed to determine change in risk behavior characteristics and to determine cumulative STD/HIV incidence. We hypothesize greater risk behavior reduction and lower disease incidence in experimental networks. In addition to its cultural relevance in Eastern Europe, the trial will inform the development of new HIV prevention strategies to reach high-risk population segments in the U.S. [unreadable] [unreadable] [unreadable]